The present invention relates to in-circuit, electrical test connectors and, more particularly, to in-circuit, electrical test connectors for surface mount technology (SMT) packages for integrated circuits and components.
SMT packages are part of an advanced, high density circuit technology presently used for the automated assembly and manufacture of printed circuit boards. SMT packages are rapidly replacing the larger dual-in-line packages (DIP's) as noted in the article by M. Chester entitled "Update: Surface-mountable Resistor Networks" appearing in the Aug. 1, 1987 issue of Electronic Products at page 19. DIP's, which are still the most popular mounting for integrated circuits and other types of components, have leads that are typically inserted by automated equipment into holes in a printed circuit board. These leads are subsequently wave soldered to the printed circuit board. This technique requires the drilling of a pattern of holes in the printed circuit board for each DIP used, which takes extra design and manufacture time. Further, the DIP's have larger, longer leads than SMT packages because the leads must structurally support the larger body of the package above the printed circuit board for convection cooling purposes. Since printed circuit boards have progressed to more compact and denser physical sizes, it has become obvious that the traditional DIP must be replaced by a smaller package, which likewise can be used with automated assembly equipment.
One of the more popular packages for automated printed circuit board assembly is the small outline (SO) SMT package. In assembly, each SO SMT package is located by the automated part locator and subsequently affixed in its respective position upon the awaiting conductor pads of the printed circuit board.
Some SO SMT packages are held in position on the printed circuit board by the solder paste which coats the leads, while other packages are held by a combination of solder paste on the leads and an adhesive between the printed circuit board and the SO SMT package. The solder paste on the leads is a special low melting temperature type which becomes molten when heated in a special temperature regulated chamber. The molten solder flows between the horizontal portions of the gull wing shaped leads of the SO SMT package and their respective conductor pads of the printed circuit board when the chamber reaches a predetermined temperature. The heat of the chamber not only solders the leads to the conductor pads, but also cures the adhesive for those SO SMT packages which are mounted with an adhesive. The soldered leads, or the soldered leads in combination with an adhesive, form a strong supportive bond between the SO SMT packages and the printed circuit board. Thus, the SO SMT packages receive structural support from the leads or the combination of the leads and the adhesive with the printed circuit board. After soldering, the SO SMT package has a clearance of approximately 0.10 mm with the printed circuit board which may or may not be occupied by the adhesive.
Because the SO SMT package is substantially smaller and lighter than its DIP counterpart, the leads can be made smaller and can be located closer together than the leads of a DIP package. Furthermore, SO SMT integrated circuits and components are typically of the low power variety which do not have to be mounted with both the top and bottom surfaces exposed for cooling. Thus, SO SMT packages are smaller and the circuits using SO SMT packages are denser, which is another way of saying the product is more compact for an equivalent functionality.
DIP's, as well as SO SMT packages, present a problem during testing, especially when in-circuit testing of a package mounted on a printed circuit board is involved. In the case of a DIP, the leads are so close together that it is necessary to use some type of test connector between the DIP-under-test and the test equipment. Otherwise, the circuit might be damaged during testing due to inadvertent connections to ground, power supplies, or other inappropriate connection. One such test connector is shown in U.S. Pat. No. 3,506,949 entitled "Electrical Connector Clip Device" by Venaleck et al., in which a number of conductors are held against the DIP leads by the combined action of a spring and two pivotally mounted jaws ending in a number of tooth-like prjections. The projections hook onto the underside of the DIP component in order to keep the clip from sliding off. Such a clip would not work on SO SMT components because the tooth-like projections do not fit between the bottom surface and the printed circuit board. The inability of this known test clip to grasp the bottom surface would prevent it from making good connections to the leads, and furthermore, would not prevent the clip from sliding off even if it were scaled down to the appropriate size.
A specialized test clip for DIP's is shown in U.S. Pat. No. 4,055,800 entitled "Test Clip for Electronic Chips" by Fisk et al. This patent also discloses a device which has a number of conductors that are held against the DIP leads by two pivotally mounted jaws. Instead of a spring, however, the jaws are held against the DIP leads by a cam. It is evident from the shape of the jaws; that the seating and locking upon the package denotes locking from the underside. As in Venaleck et al. mentioned above, the jaws cannot grasp and seat upon a SO SMT package. Thus, the only additional feature disclosed by this patent is the camming action used for pivoting the two jaws together.
The U. K. Pat. No. 1,246,101 entitled "Integrated Circuit Testing" by Trickey discloses in FIGS. 3 through 5 a different type of test connector for a DIP component as shown in FIG. 2b thereof. Instead of having two pivotally mounted jaws, this patent discloses a device having fixed side portions which guide and carry flexible, spring-like contacts. The contacts bear against the DIP leads in order to make electrical connections. The test connector is secured to the DIP integrated circuit by two, spring-actuated, pivotally mounted clips which grasp the underside of each end of the DIP integrated circuit by hook-like projections. Such a design would not work for SO SMT test connectors since the underside is generally not available for grasping.
A test connector for a FN type SMT package is shown in U. S. Pat. No. 4,671,590 entitled "Test Clip for PLCC" by M. Ignasiak. The FN type SMT package is rectangular and has leads on all four sides. This type of package has longer, J-shaped leads which curl inwardly as shown on page 7-6 of The TTL Data Book, volume 4 published in 1985 by Texas Instruments. The longer leads allow for a thicker type of adhesive mount and further provide a greater clearance between the underside of the package and the printed circuit board. This clearance is used by the test clip of this patent as a passage for the teeth which grasp the underside of the package, as shown in FIG. 4 thereof. This known type of test clip would not work on the SO SMT packages because the clearance between each SO SMT package and the circuit board is not sufficient to allow such teeth to penetrate beneath and grasp onto the bottom surface. Moreover, the design of this known test clip has four sides which angle outward in all four directions as the normal displacement above the FN type, SMT packages unit-under-test increases. Such a design is not desirable because it prevents the possibility of using two or more test clips while testing adjacent SMT packages unless the packages are fairly well spaced. Wide spacing, however, would be counter to the purpose of using the smaller SMT packages to achieve denser, more compact printed circuit boards.